Two mucopolysaccharidoses, the Hunter syndrome and the Hurler syndrome, are lysosomal disorders due to the absence, respectively, of iduronate sulfatase and iduronidase. Specific substrates have been prepared and radiological assay procedures for the purification and study of these enzymes and for the diagnosis of the corresponding genetic diseases have been developed. A corrective form of iduronidase, efficiently internalized by fibroblasts in tissue culture, has been separated from a non-corrective form by affinity chromatography.